Obviously it is very convenient to provide a multiplexed television audio signal receiver with a device for detecting whether the subchannel signal is present or absent in the television signal received and causing a display device to operate when the subchannel signal is present. When the pilot signal for the subchannel signal is also multiplexed, the display device can be turned on in response to the detection of the pilot signal, but when the pilot signal is not present, the display device is turned on in response to the detection of the carrier of the subchannel signal.
The subchannel signal is FM signal so that when the receiveer is detuned or tuned to an unused channel, a subchannel signal demodulation circuit generates the so-called FM noise which is extremely high in level. Even when the receiver is correctly tuned to a channel, the noise level rises suddenly when the antenna input drops below a certain level. In these cases, the display device such as a display lamp is turned off and the demodulated subchannel signal must be squelched or quieted. In the case of the separate-carrier type receiver, the control signal required for turning off the display device and turning on a squelch circuit can be derived from the carrier level of the main channel signal and its tuning characteristics, but in the case of the inter-carrier type receiver it is difficult to obtain such control signal. The subchannel signal is FM signal with a predetermined amplitude level. Therefore, there has been devised and demonstrated a system in which the control signal is derived by the detection of the difference in level between the subchannel signal and the noise. When the receiver is tuned to an unused channel or detuned too far, the difference is large so that the control signal can be obtained, but when the antenna input becomes weak even when tuned correctly or when the receiver is detuned only a little, the difference is less. Moreover, the prior art circuit adapted for detecting such level difference has a relatively gentle threshold level. As a result, even when the antenna input becomes weak or when the receiver is detuned so that the noise level in the demodulated subchannel signal rises, a squelch circuit will not be turned on. Furthermore, other erratic operations tend to occur very often which in turn fail to turn on the squelch circuit. Same is true for the subchannel signal display device.